The present invention concerns an apparatus for the recovery of heat or cooling from an indoor space and for its ventilation, consisting of heat accumulators in which the outgoing air releases its heat and the incoming air takes up the corresponding amount of heat, or vice versa, each accumulator being by one end alternatively connected with the exhaust and supply ducts of the room space, in which ducts the direction of flow is always the same under control by two flaps, whereas the opposite ends of the heat accumulators being correspondingly connected with the fresh air intake duct and the outgoing exhaust duct, in which latter ducts the direction of flow is also always the same under control by the same two flaps. An apparatus of this type for recovering heat from a room space and ventilating it, known in prior art, has been presented e.g. in U.S. patent application Ser. No. 438,463, now U.S. Pat. No. 4,493,366. This is intended in the first place e.g. for detached one-family houses, factory premises or hospitals. Therefore the size of the apparatus has to be dimensioned consistent with the desired effectiveness and air quantity requirements.